The Korean Waste Management Act defines “sludge” as containing less than 95% moisture or more than 5% solids. Sludges produced from municipal domestic wastewaters have similar characteristics because the properties of influent waters are within a certain range. The types of sludge include raw sludge, which is removed from the first sedimentation tank, and excess sludge (activated sludge), which is removed from the second sedimentation tank. At some sewage treatment plants, sludge is often treated together with night soil that flows into the plants.
The dehydrated cake which is finally produced at sewage treatment plants varies depending on the sludge treatment process. Generally, the amounts of raw sludge (primary sludge) and excess sludge (secondary sludge) produced from the water treatment process of a standard activated sludge method account for about 1% of the total inflow. About 40-90% of the solids within the sludge produced by this process contains organic materials, and has a moisture content of 97-99%, resulting in numerous problems in terms of disposal. That is, the large quantity of organic materials contained in the sludge is extremely unstable and prone to decomposition, and during decomposition, not only generates malodors but also can convert into substances that are hazardous to the human body and ecosystem, creating hygiene issues.
In particular, because malodor is generally composed of complex substances with multiple components, it is difficult to determine what type of malodor has how much effect on the human body, based only on the strength of malodor. It has been reported that malodors emitted from sewage treatment plants adversely affect both the mental and physical states (including respiratory, circulatory, reproductive, and endocrine systems) of nearby residents in various ways. Accordingly, it is necessary to create a turning point where the residents recognize that a sewage treatment plant is not an unwanted facility but rather an eco-friendly facility, by effectively controlling the malodors of sludge generated from sewage treatment plants.
The Korean government has pursued the “Measures for Pure Water Supply” action since the early 1990s, and has actively pushed forward public sewage treatment plant projects in regions like densely inhabited cities. As a result, about 83.5% (as of the end of 2005) of domestic wastewater is treated at the public sewage treatment plants. As of the end of 2005, a total of 294 public sewage treatment plants with a capacity of 22,568 tons have been established in Korea. The amount of sewage sludge produced in Korea is generally 0.3 kg per 1 m3 of sewage, where the moisture content is about 78% and the organic material content is about 47%. When converted to a daily sewage amount, this corresponds to 6,770 tons of sewage sludge per day.
Recently, studies relating to methods for decomposing malodorants (mainly, organic materials) via enzymatic reactions by microorganisms are being actively carried out in advanced countries including the U.S. Microorganisms that can be used for the biological decomposition of malodorants can be largely divided into two groups—autotrophic bacteria and heterotrophic bacteria. Autotrophic bacteria are microorganisms which can grow only with inorganic materials; for example, sulfur bacteria can synthesize the necessary complex organic materials in the presence of only a small amount of inorganic salts, water and carbon dioxide. Accordingly, autotrophic bacteria are useful for the conversion of inorganic materials. Nonetheless, due to their slow energy conversion process and low growth rate, there have been no reports so far of a case where autotrophic bacteria have been successively used for the conversion of inorganic materials. Moreover, it is known that this type of bacteria can be applied to only a few simple processes.
Meanwhile, heterotrophic bacteria are microorganisms which obtain nutrients and energy necessary for growth by decomposing a carbon source in the form of an organism and other organic materials. Although such heterotrophic bacteria are particularly well adaptable to the conversion of organic contaminants, it is very difficult to enumerate every bacterium suitable for the optimal conversion of a specific contaminant. However, the conversion process by those heterotrophic bacteria is much faster than that by autotrophic bacteria. Since the conversion of organic compounds by heterotrophic bacteria requires oxygen, in most cases, the introduction of oxygen and bacteria acts as a limiting step.
Based on the above, in removing malodors produced from sewage treatment plant sludge, a biological process using microorganisms is considered as the most economical and eco-friendly method with no secondary contamination. Thus, in order to remove malodors produced from sewage treatment plant sludge, the inventors of the present invention developed a deodorizing apparatus using native microorganisms and attempted to solve the problems relating to malodors generated from the primary sludge, secondary sludge, primary sludge containing night soil, etc. produced from the sewage treatment plant, and from the concentration tank, dehydrator, dehydrated cake, etc. of the sewage treatment plant.